At present, a handheld terminal device is typically provided with a plurality of frequency bands, for example, frequency bands required for the Global System for Mobile communications (GSM) and the Digital Cellular System (DCS) of a mobile phone (GSM+DCS) as well as an Ultra High Frequency (UHF) and a frequency of the Global Positioning System (GPS) of an interphone, etc., to enable a plurality of functions or auxiliary functions, wherein dual- or multi-frequencies antenna corresponding to the plurality of frequency bands is provided. In the prior art, it is common to adopt a dual frequency antenna in a structure of partial resonance in which a higher frequency band is designed with different structural parameters so that one frequency is generated throughout an antenna dipole while high frequency resonance arises from that a helix part with different parameters. In an early mobile phone antenna, for example, the DCS frequency band is typically placed at the bottom of a coil for handling.
Numerous dual frequency antennas operate in an operation mode of UHF+GPS frequency bands, which is typically implemented with partial resonance of a helical structure in a way that its part of high-frequency resonance is placed at the bottom of a coil and constitutes lower-frequency resonance together with the other part. Reference is made to FIG. 1 illustrating a schematic structural diagram of a dual frequency antenna with partial resonance in the prior art in which its part of GPS resonance is placed at the bottom of a helix to form resonance. For the GPS frequency band, good performance of the antenna is concentrated largely at the lower half of a spherical surface while poor performance is at the upper half of the spherical surface required for the GPS (its part pointing to the sky), which is not suitable for specialized GPS performance and functional positioning of specialized terminal devices.
The entire antenna can be tuned easily only if a frequency at which the antenna operating in the GPS frequency band is an odd multiple (e.g., one, three, five, seven, etc., times) of that in the UHF frequency band or otherwise might be difficult to tune in any other frequency band. For example, an external dual frequency antenna of an existing interphone operates in an operation mode of UHF+GPS frequency bands in which the entire Ultra High Frequency (UHF) band ranges from 300 to 870 MHz. When the frequency of GPS resonance is that of a three-order resonance relative to the UHF frequency band (five times of dominant frequency), easy tuning is possible only if the UHF frequency is approximately one fifth of 1575 MHz or otherwise might be difficult in any other frequency band and is almost impossible, let alone accurate tuning, especially at 3.5, 4.5, 5.5, etc., times of the frequency. Consequently, it may be inconvenient in the prior art to tune the GPS+UHF operating dual frequency antenna in some frequency bands, thus adverse to transmit and receive a signal in a plurality of frequency bands by the antenna.